1. Field of the Invention
This invention relates generally to a liquid crystalline composition, and, more particularly, it is concerned with such liquid crystalline compositions which have extended ranges of the nematic liquid mesomorphic temperature, are stable at room temperature, and have an applied use in the field of electro-optical display devices.
2. Description of the Prior Art
As is well known, there are three types of liquid crystals depending on their molecular orientation, i.e., nematic, smectic, and cholesteric. Of these, development in the nematic liquid crystal material for use in electro-optical display devices has been very active in recent years.
The nematic liquid crystal is classified into two groups according to the dielectric property thereof. The first group in an Nn type liquid crystal having a negative dielectric anisotropy, wherein such dielectric anisotropy lies in the direction substantially perpendicular to the molecular axis. The second group is an Np type liquid crystal having a positive dielectric anisotropy, wherein such dielectric anisotropy is substantially parallel with the molecular axis.
The Nn type liquid crystal has been the focus of research and development since it was applied to a panel type display device utilizing its dynamic scattering mode (DSM) which was discovered by Heilmeier of R.C.A. in 1968, and it is now practically commercialized in horological instruments, desk-type electronic calculators, and so forth.
On the other hand, the Np type liquid crystal, on account of its substantial inability to exhibit the dynamic scattering mode, has been considerably behind in its development. However, it has begun to draw the attention of many experts in this field since M. Schadt and W. Helfrich of F. Hoffmann-La Roche & Co. published in "Applied Physics Letter," Vol. 18(4), p 127, 1971 a paper concerning the twisted nematic mode (TNM).
While the method of manufacturing a liquid crystal cell for display devices utilizing the Np type liquid crystal exhibiting such twisted nematic mode is reported in detail in the abovementioned literature as well as described in published Japanese patent application No. 47-11737, it can be outlined as follows.
The liquid crystal cell is of such construction that the Np type liquid crystal material is interposed between a pair of electrodes through a spacer, at least one of which is transparent, wherein the electrode surfaces are so orientation-treated as to cause the molecular axes of the liquid crystal material to be within the electrode surfaces, and to cause the molecular axes of the liquid crystal arranged on each electrode surface to be perpendicular to each other, and the molecular axes are in parallel with the electrode surfaces and form a spiral at an angle of 90.degree. from one of the surfaces to another. It is, of course, possible that this spiral angle can be freely varied from 0 to 90 degrees by changing the direction of the orientation. This liquid crystal cell is further provided with a polarization plate to enable the same to be used for display purposes.
The principle of display utilizing this twisted nematic mode of the Np type liquid crystal can be explained in the following way. In the case of the 90 degree spiral, when the an electric field is not being applied to the cell, linear polarized light which has been vertically projected onto the electrode surface comes out of the liquid crystal cell as the result of the plane of the polarization being turned 90 degrees. Therefore, when this liquid crystal cell is placed between a cross-nicol prism to observe the transmitted light through it, the sight looks bright, and, when the cell is placed between a parallel-nicol prism for observation, the sight is seen dark. In this state of the liquid crystal cell, when an appropriate electric voltage (saturated voltage) is impressed on this cell, the molecular orientation is carried out in such a way that the long (or major) axis of the liquid crystal molecule becomes perpendicular to the electrode surface, so that, opposite to the case where no voltage is impressed, the sight is seen dark in the case of the observation through the cross-nicol prism, and the sight is seen bright in the case of the observation through the parallel-nicol prism. Accordingly, a required pattern display can be achieved by the contrast between the bright portion of the sight and the dark portion of the same.
This twisted nematic mode (TNM) has a quick response, operates at a low voltage, is small in its electric power consumption, and has some other advantages as compared with, for example the conventional dynamic scattering mode (DSM), on account of which this mode is indispensable in the application of the liquid crystal to the display devices. As a consequence, development in this Np type liquid crystal to be used for the foregoing purposes has become more and more active in recent years.
However, Np type liquid crystals of benzylidene, azo, azoxy, and carboxylic acid ester series which have been heretofore known have several disadvantages, owing to which they have not yet been proved satisfactory in practical use. The reason for this can be summarized as follows.
(1) At first, there exists a temperature range between a transition temperature T.sub.N where the material shifts from a crystal or smectic liquid crystal phase to the nematic liquid crystal phase and a transition temperature T.sub.I where the material shifts from the nematic liquid phase to an istropic liquid, i.e., a temperature range where the material exhibits the nematic liquid crystal phase (hereinafter referred to as "mesomorphic range" or simply "M.R.") at a higher temperature side. Accordingly, the material possesses high transition temperature T.sub.N, and does not exhibit the nematic liquid crystal phase at a room temperature or a lower temperature. For instance, liquid crystal materials of the carboxylic acid ester type have their M.R. almost deviated to the high temperature side of as high as 60.degree. to 200.degree. C., hence they lack practability. Also, in the liquid crystal materials of the azoxy type, there have so far been none that are capable of exhibiting the nematic liquid crystal phase at a normal temperature in its single form (most of these materials have the lower limit of M.R., that is, T.sub. N of as high as 100.degree. C. or so). Even in the mixed liquid crystals of the azoxy type, there are only a few having M.R. of from 0.degree. to 60.degree. C. or so. On account of this, when this type of liquid crystal is to be used, it has been the usual practice to admix it with an Nn type low temperature liquid crystal such as, for example, p-methoxybenzylidene-p'-n-butylaniline (having M.R. of 20.degree. to 40.degree. C.) so as to broaden their M.R. to a low temperature region. However, the Np type liquid crystalline composition, in which the Nn type liquid crystal is incorporated, has been found to be slower in its response than that of the liquid crystalline composition consisting of the Np type liquid crystal alone, or a mixture of Np type liquid crystal materials, and to have other disadvantages in its electrical characteristics.
Further, in the general liquid crystal material, there is the so-called overcooling phenomenon, wherein the liquid crystal phase can be maintained even when the temperature is decreased lower than the lower limit temperature T.sub.N of M.R. If this degree of overcooling is not sufficiently high, the device utilizing such liquid crystal therein becomes inoperative when, for example, it is cooled below the lower limit temperature of the M.R. of the liquid crystal through carelessness or by an unexpected event to solidify the liquid crystal. On the contrary, however, when the degree of the overcooling is sufficiently high, the liquid crystal remains unsolidified, even if the liquid crystal is cooled below the lower limit temperature T.sub.N of the M.R., so that its electro-optical property within the M.R. can be retained as is. In view of such facts, development of a liquid crystal material having a high degree of overcooling has been desired.
(2) Secondly, a field-effect type cell utilizing the abovementioned Np type liquid crystal is capable of being driven by a relatively lower voltage than the liquid crystal cell utilizing the dynamic scattering mode (usually at 3 to 6 volts), and is rather difficult to cause electric current to flow therethrough, hence its service life can be relatively prolonged. However, on account of its being affected by the electrochemical reactions and ultraviolet rays, the cell has been rather short in life in comparison with the conventional Nixie tube, L.E.D., and so forth.
(3) Thirdly, the liquid crystal cell of this type becomes excellent in its electrical performance according as the dielectric anisotropy is larger. However, in the conventional liquid crystal material, its dielectric anisotropy was from 0 to +3 or so, the value of which was not sufficient for the practical use of the liquid crystal cell.
(4) Fourthly, the liquid crystal material is generally of high viscosity, which largely affects the voltage impression, and response at the time of elimination, particularly the trailing at the time of the elimination and it is poor in the response upon pulse drive and is not suitable for multiple place dynamic drive. Therefore, the liquid crystal material having a low viscosity has been desired.
The afore-described biphenyl type liquid crystal material of Np type has been found to solve various problems inherent in the conventional liquid crystal material, and was published in "Electronics Letter," Vol. 9(5), Mar. 8, 1973.
This liquid crystal material represented by the following general formula, ##STR1## in comparison with the conventional liquid crystal material, is very stable chemically, since the material has no functional group between the two phenyl groups. Also, this liquid crystal material has a large dielectric anisotropy of as high as +10, is lower in its torsional viscosity than that of the conventional liquid crystal, exhibits the liquid crystal phase at a relatively low temperature, and possesses a high degree of overcooling. However, even in such a favorable liquid crystal, its mesomorphic range (M.R.) is as narrow as from -2.degree. C. to +37.degree. C., and, moreover, the range is deviated to a lower temperature side (normally, the M.R. of from -10.degree. C. to +60.degree. C. is necessary), so that the material is still not satisfactory from a practical point of view. Such narrow mesomorphic range naturally restricts the useful temperature range with the consequence that the marketing area for such a product is limited, hence their value as a commercial article inevitably lowers. In another aspect, in the alpha-numerical display, there are two systems of static drive and dynamic drive for the reason of the lead wire connection, and, where the number of characters to be displayed is large, the dynamic drive system has been known to be very advantageous. In this case, however, due to the narrow range of mesomorphic temperature of this liquid crystal material, the power source voltage has to be varied in compliance with variations in the ambient temperature, hence it has so far not been possible to fully adopt the dynamic drive system.
In order to conduct dynamic drive (time sharing), a voltage is not applied continuously to a segment to be displayed, but it is periodically applied to the segment at a short pulse width. The liquid crystal material to be utilized for a liquid crystal display element is required to exhibit appropriately a response to the short pulse width. Specifically, the ratio of the time for applying a voltage onto one segment to the pulse repetition time or one cycle time within the duration of signal input is called the duty ratio. A smaller duty ratio stands for a shorter voltage applying duration within one pulse repetition time or one cycle time. Consequently, the dynamic drive can be made possible only by utilizing the liquid crystal material which is capable of exhibiting a good response under the condition of a small duty ratio.
Where the ratio of a voltage applying duration (ON) to a no-voltage applying duration (OFF) is 1:1, the duty ratio is 1/2. In this case, the voltage is applied to the segment for the half of the signal input duration (one cycle time), and at this time, when the signal input is effected cyclicly, it becomes possible theoretically to conduct 2-place dynamic drive. In the use of the conventional liquid crystal having a negative dielectric anisotropy, it is hardly possible to make the duty ratio smaller, and therefore, the dynamic drive system has been inoperable so far. Even by the use of the conventional liquid crystal having a positive dielectric anisotropy, only 2-place dynamic drive has been operable.
To put the above in more detail, the dynamic drive is theoretically the same as the matrix drive as shown in FIG. 1. Generally speaking, in liquid crystal cells, the way of impressing voltage is carried out as shown in FIG. 1 so as to prevent undesirable cross-talk. In this drawing, the vertical and horizontal wires to denote the electrodes are shown to intersect each other, although in the actual construction of the liquid crystal display cell, they are spaced apart at each of the crossing points, in which space the liquid crystal material is filled.
In FIG. 1, the black dots denote the selection points, to which a voltage V is applied, while the white dots represent the semi-selection or non-selection points, to which a voltage 1/3V is applied. The voltage V is so determined that these black dot portions may be subjected to scattering, and the white dot portions may not be subjected to scattering.
FIG. 2 indicates the relationship between electric voltage and percent light transmission (hereinafter called "transmission"). In this case, if a voltage V which is sufficiently larger than the voltage value Vs at the time of the transmission reaching its point of saturation can be used, there occurs no serious problem in the contrast, etc. However, as mentioned in the foregoing, there inevitably takes place cross-talk, which compels the restrictive upper limit voltage Vb to be set. On the other hand, as it is necessary to obtain a certain definite transmission, a lower limit voltage Va is also required for such purpose. In other words, when the matrix drive is to be carried out by utilizing the liquid crystal cell, the power source voltage V should be so designed as to satisfy the relationship Va.ltoreq.V.ltoreq.Vb. In this case, however, as this "Vb minus Va" relationship (which will be referred to hereinafter as "voltage margin" or "marginal voltage") is as small as from 0.1 to 0.2V or so, the circuit designing becomes very complex and expensive. In addition, as there is another disadvantage of the voltage margin being considerably varied by the temperature, the useful temperature range is naturally restricted even in this aspect, from which point the value of this liquid crystal cell is still poor as a commercial article.
More amplified explanations of the foregoing will be given in the following with reference to FIGS. 3(a) and 3(b). FIG. 3(a) shows the relationship between temperature and voltage margin within the mesomorphic range (M.R.), and FIG. 3(b) indicates the same relationship in the typical example of an ordinary nematic liquid crystal.
In FIG. 3(a), when the temperature range for driving the liquid crystal cell is set between T.sub.1 and T.sub.2, the liquid crystalline composition becomes excellent as the portion S shaded with oblique lines in the drawing becomes larger. However, if, as shown in FIG. 3(b), the lower limit voltage Va at T.sub.1 is greater than the upper limit voltage Vb at T.sub.2, there exists no part at all corresponding to the above-mentioned portion S, so that, when the driving temperature range T.sub.1 -T.sub.2 is given preference, it become necessary that the circuit should be so designed as to cause the voltage to vary depending on the temperature variations. Also, if the voltage is made constant, the operable temperature range of the device which utilizes the liquid crystal cell should be restricted. As a consequence, not only will the marketing area of the product cell be naturally limited, but also there are various disadvantages in the production cost or sales of such a product. For the above-mentioned reasons, development of the liquid crystalline compound having sufficiently large mesomorphic range (M.R.) with respect to the driving temperature range T.sub.1 -T.sub.2 has been demanded.